The invention relates to a catalyst comprising a spinel containing copper, zinc and aluminum, and a process for its preparation.
German patent document DE 43 01 470 Al (which is not a prior publication) discloses combining a CuAl.sub.2 O.sub.4 spinel with tin, lead, an element of the main or transition group II of the Periodic Table of the Elements as oxide or salt or in elemental form and subsequent calcination, to yield a spinel. The previously known spinel of this type is used for the decomposition of N.sub.2 O. The use of this catalyst for catalytic oxidation of CO.sub.2 and NO.sub.x, in particular at temperatures of above 100.degree. C., is not known.
For reasons of environmental protection, the purification of gases, in particular exhaust gases as they are formed (for example, in internal combustion engines, particularly diesel or lean-mix engines), in addition to lowering CO concentration, etc., to remove nitrogen oxides, i.e. the degradation of NO.sub.x, is an urgent problem.
European patent document EP 042 471 B1 discloses a catalyst used for the catalytic oxidation of carbon monoxide to carbon dioxide (CO.sub.2), which catalyst contains the metals copper, zinc and aluminum as metal oxides. At least 60% of the Cu present is bound to the aluminum oxide as a copper oxide-aluminum oxide spinel. In the free spaces of the porous spinel, zinc oxide is arranged in a proportion by weight of from 1% to 20%.
However, thus catalyst is known to have a purifying action only in respect of CO, which is based on a catalytic oxidation of the CO. Whether the catalyst is likewise suitable for purifying gases containing NO.sub.x and/or hydrocarbons, as formed, in particular in the above mentioned internal combustion engines and, for example, also in thermal power stations and the like, is not known.
It is an object of the present invention to provide a catalyst of the generic type which has a good purification action, in particular for NO.sub.x, together with as good as possible a stability to the constituents of waste gases.
This object is achieved by the catalyst and method according to the invention, which provides a support material of zeolite with which the spinel is mixed or to which it is applied. The use of a Cu.sub.A Zn.sub.C Al.sub.D O.sub.4 spinel and a zeolitic support material as a catalyst makes the catalyst largely stable to the constituents of the waste gases such as, for example, H.sub.2 O, NO.sub.x, CO.sub.2 and/or SO.sub.2, with the catalyst having an oxidizing action on hydrocarbons and a catalytically reducing action on the NO.sub.x. The reduction of NO.sub.x takes place in oxygen-containing gas and in the presence of reducing agents such as, for example, hydrocarbons just mentioned. Advantageously, hydrocarbons are present in exhaust gases of internal combustion engines in sufficient concentration. In favorable cases, reductions of above 60% can be achieved at temperatures greater than 300.degree. C.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.